Quicksand

This article is about the geological feature. For other uses, see Quicksand (disambiguation).

Quicksand and warning sign at a gravel extraction site.

Quicksand is a colloid hydrogel consisting of fine granular matter (such as sand or silt), clay, and salt water. The origin of the name refers to "quick" in the older meaning of "alive" rather than "fast," and is thus similar to the origin of the term quicksilver for mercury.

Basically, water circulation underground can focus in an area with just the right mixture of fine sands and other materials such as clay. The water moves up and then down slowly in a convection-like manner throughout a column of sand under optimal conditions, and the sand remains a generally solid mass. This lubricates the sand particles and renders them unable to support any significant weight, since they move around with very little friction, behaving more like a liquid when exposed to stress. Since the water does not usually go all the way up through the sand, the sand above does not appear to move at all, and can support leaves and other small debris, making quicksand difficult to distinguish from the surrounding environment.

Properties

Quicksand is a non-Newtonian fluid: when undisturbed it often appears to be solid ("gel" form), but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form).

After the initial perturbation—such as a person attempting to walk on it—the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to suddenly increase. If a person steps in it they will start to sink. In order to move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of one centimeter per second would require the same amount of force as "that needed to lift a medium-sized car." "A. Khaldoun, E. Eiser, G. H. Wegdam and Daniel Bonn Rheology: Liquefaction of quicksand under stress" \'Nature\' Vol. 437, Pg. 635, 29 September 2005 doi:10.1038/437635a

Recent research findings

It was commonly believed that the behavior of quicksand was due solely to saturated or supersaturated suspensions of granules in water. Pressure from underground sources of water would separate and suspend the granular particles, reducing the friction between them. As of September 2005, it has been shown that it is the presence of salt that is largely responsible. The stability of the colloidal quicksand is compromised by the presence of salt, increasing the likelihood of sand flocculation and the formation of the high viscosity regions of sediment responsible for quicksand\'s "trapping" power.

Prevalence

Quicksand can be found inland (on riverbanks, near lakes, or in marshes) or near the coast.

One region notorious for its quicksands is Morecambe Bay, England. As the bay is very broad and shallow, a person trapped by the quicksand would be exposed to the danger of the returning tide, which can come in rapidly.

Despite the dangers of quicksand, the TV shows Brainiac and MythBusters proved it was impossible for a human to be drowned in quicksand, and that victims died for some other reason.

See also

• Dry quicksand
• Fech fech
• Liquid limit
• Plastic limit
• Soil liquefaction
• Thixotropy

References

External links

• Howstuffworks.com - How quicksand works
• "What is quicksand?", Scientific American. 
• Quicksand in Fiction
• Brainiac on Quicksand (Youtube)

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